Cadmium red-based pigments, and enamels comprising those pigments in combination with pigment-supporting fluxes, are well known in the art. Cadmium red pigment is considered to be a mixed CdS/CdSe crystal, commonly referred to as cadmium sulfoselenide, wherein composition variations can provide red coloration within a considerable range of color.
As noted by C. W. Parmelee in Ceramic Glazes, p. 447, Third Edition, Cahners, Boston, Massachusetts, (1973), cadmium red pigments are unstable at temperatures above about cone 015-010, and provide dependable coloration only with short duration firing. Thin coloring layers are particularly sensitive to overheating, although coloring variations can be controlled somewhat through the use of thicker coloring layers.
The color of cadmium red pigments can be stabilized through the use of cadmium-containing fluxes. U.S. Pat. No. 3,857,746 to Blanco et al., for example, reports the use of a CdO--PbO--B.sub.2 O.sub.3 --SiO.sub.2 flux in combination with a CdS/CdSe pigment to provide enamels for decals. Even where cadmium red enamels comprising cadmium-containing fluxes are used, however, the color stability of the resulting enamels during firing is relatively poor. Thus these colors have a strong tendency to turn gray at firing temperatures over about 800.degree. C., with soak times in excess of about 10 minutes, so that relatively mild firing treatments must be used in the application of cadmium red decorations.
The combined use of cadmium-containing fluxes and relatively low firing temperatures produces a fired decoration with rather poor chemical durability. Specifically, the fired decoration has low resistance to attack by alkaline media such as the commonly used detergents.
It is a principal object of the present invention to provide cadmium red decoration systems using CdS/CdSe pigments exhibiting improved resistance to color burn-out at high temperatures.
It is a further object of the invention to provide ceramic articles decorated with cadmium red-based decorating enamels which exhibit improved chemical durability.
Other objects and advantages of the invention will become apparent from the following description thereof.